The present invention relates to an optical information recording disk, and in particular, to a so-called air-sandwich optical disk in which a signal recording layer faces a cavity inside the disk.
One type of air-sandwich optical disk is a write-once type formed by disk-like substrates (made of a plastic material such as PMMA or the like), signal recording layers (made of, for example, an organic pigment film and) carried on main surfaces of the disk-like substrates, and annular spacers on which the substrates are mounted so as to face each other. Such a disk is generally assembled using the method shown in FIGS. 1(a) through 1(f).
First, as shown in FIG. 1(a), a pair of ring-like or annular inner and outer circumferential spacers 1 and 2, respectively, are centered and fixed onto a positioning jig, and an adhesive agent 3 is applied to all substrate bearing surfaces of the spacers 1 and 2.
Next, as shown in FIG. 1(b), a substrate 5 carrying on its main surface a signal recording layer 4 is aligned with a central hole in the inner spacer 1 and is mounted on the spacers 1 and 2 with the signal recording layer 4 facing the spacers. The substrate 5 is pressed downward slightly so that any gaps between the substrate 5 and each of the spacers 1 and 2 are filled with the adhesive agent 3.
Next, as shown in FIG. 1(c), the substrate 5, together with the spacers 1 and 2, is turned over and fixed onto the jig, and the adhesive agent 3 is applied to the spacers 1 and 2 in the same manner as in the first step shown in FIG. 1(a).
As shown in FIG. 1(d), another substrate 6 is then mounted on the spacers 1 and 2 in the same manner as in the step shown in FIG. 1(b). The assembly is left for a predetermined time at room temperature. Then, the assembly is heated for two hours at 60.degree. C., and left again for a predetermined time at room temperature so that the adhesive agent hardens.
After hardening of the adhesive agent, as shown in FIG. 1(e), the outermost circumferential portion of the disk, that is, the circumferential portions of the disk-like substrates 5 and 6 and the outer circumferential surfaces of the outer spacer 2, are finished to form a uniform radius from the center of the disk.
Then, as shown in FIG. 1(f), the disk is finished into an optical disk 7.
The air-sandwich optical disk is formed using acrylic plates of PMMA or the like as the substrates, and using materials such as cyanine organic pigment films or metal thin films of tellurium as the recording films. The adhesive agent must be such that it does not damage the recording films, yet has a suitable adhesive strength with the acrylic plates. The adhesive agent must also seal the disk so that moisture cannot get into the inside space, and must have a viscoelasticity so as to accommodate warp (e.g. from water absorption of the acrylic plates). One such adhesive agent is a thermosetting polyurethane. Accordingly, in the joining step shown in FIG. 1(d), the whole disk is heated, so that the air in the interior space expands. The expanding air causes transformation of the substrates. The expanding air also penetrates and enters the adhesive agent layer (which has not yet hardened) and lowers the adhesive strength. To prevent transformation of the substrates and mixture of air bubbles in the adhesive agent layer from occurring, an air hole 10 is formed to allow the expanding air to escape (see Japanese Utility Model Unexamined Publication No. 57-149535), as shown in FIG. 2. The air hole 10 is shown in the outer circumferential spacer 2, but it could be provided in the substrates or the inner circumferential spacer 1. It serves to equalize the atmospheric pressure between the air in the interior space and the air outside the disk so as to prevent warping of the substrates owing to a change in humidity, or the like.
To ensure that only air passes through the air hole 10 (i.e. to prevent dust or the like from entering), a filter covers the hole. However, even when such a filter is used, ozone, sulfurized gas, and the like, may enter through the filter from the ambient air. The entering gases will damage the signal recording films. For example, a signal recording film made of organic pigment films can be decolorized, and a metal film signal recording film can be oxidized, rendering it impossible to maintain desired properties of the signal recording film.
In addition, in the conventional air-sandwich optical disk, the thickness of the interior space, that is, the thickness of the spacers, must be sufficiently large so that the air hole can be formed in the spacers. However, thin optical disks have been developed with the miniaturization of recording/reproducing apparatus. For example, in an optical disk which is 3 mm thick and in which each substrate is 1.2 mm thick, a sufficiently large air hole cannot be formed because each spacer is only 0.6 mm thick, whether or not the air hole is formed in the spacers. Further, in some cases the air hole may be filled up with adhesive agent when the spacers and the substrates are adhered to each other. In such cases, the air hole cannot function as a path for expanding gas.